This invention relates to electrostatography, and more particularly to a method and apparatus for removing heat from the pressure roller of the fuser assembly of an electrostatographic apparatus.
In the practice of xerography as described in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.
The latent electrostatic image may then be developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited; and where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably affixed to thereby form a permanent print.
The toner may be fixed by passing the sheet of paper or other surface including the transferred image between a heated roller and a second roller in pressure contact therewith whereby the toner becomes fused to the sheet of paper. In accordance with such techniques, the temperature to which the toner is generally raised is a temperature (for a given pressure) at which the toner particles coalesce or flow together and wet the paper surface to effect thereby a permanent bond. The heated or fuser roller is generally a sleeve formed of a heat conductive material, for example, copper, whereas the contact or pressure roller is a sleeve having a thick outer layer of a resilient material, such as rubber coated with a protective layer of a heat resistant material, e.g., Teflon (available from Du Pont). When the two rollers are placed in pressure contact, a nip of sufficient width is developed to fix efficaciously the toner to the image receiving member. In the electrostatographic machine hereinafter described, the image receiving member on which the developed electrostatographic image is to be transferred, such as paper, is passed through the fusing zone sideways as compared to lengthwise as in most prior devices, i.e. line copy is perpendicular to the axis of the rollers. The temperature to which the heated or fuser roller is raised is sufficiently high to effect deleteriously any portion of the pressure roller which is in continuous contact with the heated fuser roller (i.e. end portions of the pressure roller where image receiving members being processed are of a dimension less than the designed maximum dimension therefor), there being sufficient heat removal in those areas wherein image receiving members are in contact with the rollers.